Current aims for the project include: 1) exploration of the role which membrane potentials play in function of Na ion-dependent transport systems for organic solutes; 2) evaluation of energetic sufficiency of transmembrane electrochemical gradient for Na ion for maintaining steady gradients of sugars observed in isolated enterocytes, and 3) determination of the functional polarity of the enterocyte for transepithelial movement of sugars and amino acids. The cell to medium distribution of fluorescent cationic dyes is being used as a means of detecting both qualitative and quantitative changes in membrane potential in situations where transport of organic solutes can be monitored simultaneously. Selective alterations in membrane potential are induced with the use of specific ionophores and varying concentrations of appropriate cations. Values determined for cellular sugar and Na ion concentrations, along with those for membrane potentials in various situations, allow a comparison between calculated maximum theoretical solute accumulation ratios, and actual observed ratios. Another approach entails the preparation of phospholipid vesicles in which elements of intestinal brush border membranes have been introduced. Selective manipulation of the potential across these vesicular membranes, allows the effect of potential on functional capability of the Na ion-dependent transport components to be evaluated.